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Installers are built and hosted by us — the one deliberately centralized piece of an otherwise decentralized app, since somebody has to publish it. Once installed, updates apply automatically.
Linux
AppImage auto-updates in place, same as Windows. The .deb/.rpm packages are a regular package-manager install — update by re-downloading.
Android: enable unknown sources
Since this isn't distributed through the Play Store, Android will ask you to confirm installing from an unknown source the first time — that's expected. You only need to allow it once.